Method and apparatus for robust tuning of model-based process controllers used with uncertain multiple-input, multiple-output (MIMO) processes

ABSTRACT

A method includes obtaining information identifying (i) uncertainties associated with multiple time-domain parameters of a model and (ii) time-domain performance specifications for a model-based industrial process controller. The model mathematically represents a MIMO industrial process. The method also includes generating multiple tuning parameters for the controller based on the uncertainties and the time-domain performance specifications. The tuning parameters include vectors of tuning parameters associated with the controller, and each vector includes values associated with different outputs of the industrial process. The time-domain parameters could include a process gain, a time constant, and a time delay for each input-output pair of the model. The time-domain performance specifications could include requirements related to worst-case overshoots, settling times, and total variations. The uncertainties could be specified as intervals in which the time-domain parameters lie.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.14/314,221 filed on Jun. 25, 2014 and entitled “METHOD AND APPARATUS FORSPECIFYING AND VISUALIZING ROBUST TUNING OF MODEL-BASED CONTROLLERS,”which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to industrial process control systems.More specifically, this disclosure relates to a method and apparatus forrobust tuning of model-based process controllers used with uncertainmultiple-input, multiple-output (MIMO) processes.

BACKGROUND

Model predictive control (MPC) techniques use one or more models topredict the future behavior of an industrial process. Control signalsfor adjusting the industrial process are then generated based on thepredicted behavior. MPC techniques have become widely accepted invarious industries, such as the oil and gas, pulp and paper, foodprocessing, and chemical industries.

When tuning an MPC or other model-based process controller forindustrial use, it is often necessary or desirable to find tuningparameters that ensure good controller performance in spite of both (i)process disturbances and (ii) mismatches between a model used by thecontroller and the actual process. This problem falls into thediscipline of “control theory” and the practice known as “robustcontrol.” Standard robust control techniques use a concept known as“unstructured uncertainty,” which generally involves analyzing andspecifying performance in the frequency domain. These robust controltechniques have been used to successfully tune process controllers in avariety of industries.

SUMMARY

This disclosure provides a method and apparatus for robust tuning ofmodel-based process controllers used with uncertain multiple-input,multiple-output (MIMO) processes.

In a first embodiment, a method includes obtaining informationidentifying (i) uncertainties associated with multiple time-domainparameters of a model and (ii) time-domain performance specificationsfor a model-based industrial process controller. The modelmathematically represents a MIMO industrial process. The method alsoincludes generating multiple tuning parameters for the controller basedon the uncertainties and the time-domain performance specifications. Thetuning parameters include vectors of tuning parameters associated withthe controller, and each vector includes values associated withdifferent outputs of the industrial process.

In a second embodiment, an apparatus includes at least one memoryconfigured to store information identifying (i) uncertainties associatedwith multiple time-domain parameters of a model and (ii) time-domainperformance specifications for a model-based industrial processcontroller. The model mathematically represents a MIMO industrialprocess. The apparatus also includes at least one processing deviceconfigured to generate multiple tuning parameters for the controllerbased on the uncertainties and the time-domain performancespecifications. The tuning parameters include vectors of tuningparameters associated with the controller, and each vector includesvalues associated with different outputs of the industrial process.

In a third embodiment, a non-transitory computer readable mediumcontains instructions that when executed cause at least one processingdevice to obtain information identifying (i) uncertainties associatedwith multiple time-domain parameters of a model and (ii) time-domainperformance specifications for a model-based industrial processcontroller. The model mathematically represents a MIMO industrialprocess. The instructions when executed also cause at least oneprocessing device to generate multiple tuning parameters for thecontroller based on the uncertainties and the time-domain performancespecifications. The tuning parameters include vectors of tuningparameters associated with the controller, and each vector includesvalues associated with different outputs of the industrial process.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates an example web manufacturing or processing systemaccording to this disclosure;

FIG. 2 illustrates an example internal model control structure employedfor model predictive control (MPC) of an uncertain multiple-input,multiple-output (MIMO) process according to this disclosure; and

FIGS. 3A through 5B illustrate details of an example technique forrobust tuning of a model-based process controller used with an uncertainMIMO process according to this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 5B, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the invention may be implemented inany type of suitably arranged device or system.

As noted above, standard robust control techniques have been used tosuccessfully tune model predictive control (MPC) and other model-basedprocess controllers in a variety of industries. However, standard robustcontrol techniques often rely upon highly-trained andhighly-knowledgeable personnel. This often makes it more difficult andexpensive to tune model-based process controllers.

This disclosure provides techniques that support robust tuning of MPCand other model-based controllers that are used with multiple-input,multiple-output (MIMO) multivariable processes having model uncertainty.A specific example described below involves robust tuning ofmachine-direction (MD) MPC controllers for paper or other webmanufacturing or processing systems. Among other things, thesetechniques can be used to achieve satisfactory closed-loop responsesthat are quantified by characteristics such as overshoots, totalvariations, and settling times under parametric uncertainty within alimited tuning time. An efficient visualization technique for anuncertain MIMO process is described to characterize the set of possiblestep responses for outputs of the MIMO process due to the parametricuncertainty. The tuning problem can be formulated as an optimizationproblem with an implicit objective function and constraints. Based onthe visualization technique and properties of performance indices, anautomatic tuning algorithm is described to solve the optimizationproblem. In addition, a technique to predict the computation time of thetuning algorithm is described, which could be used to improveuser-friendliness of the overall algorithm.

Depending on the implementation, the techniques disclosed in this patentdocument allow for the robust tuning of model-based process controllers.Example features of the techniques can include:

(1) the specification of model uncertainty in terms of simple modelparameters;

(2) the specification of controller performance using simple time-domainconcepts (such as settling time, overshoot, and total variation) foreach process output;

(3) the visualization of robust performance through step and disturbanceresponse plots with envelopes of possible responses given theuncertainty of a model;

(4) an algorithm (such as a MATLAB algorithm) that takes user-friendlyspecifications and returns appropriate tuning parameters for a MIMOmodel-based process controller; and

(5) an algorithm (such as a MATLAB algorithm) that provides robustperformance envelope visualization for a multivariable process.

In some embodiments, a user interface can be provided that allows usersto enter model uncertainty specifications and performancespecifications. The user interface can also allow users to viewresulting tuning parameters and visualize the resulting controllerperformance.

Note that in the following description, an example of this functionalityis given with respect to use with a controller in a paper manufacturingsystem. However, this disclosure is not limited to use with controllersin paper manufacturing systems. The techniques disclosed in this patentdocument can be used with any suitable model-based controller that isused to control any aspect(s) of an uncertain MIMO process.

FIG. 1 illustrates an example web manufacturing or processing system 100according to this disclosure. As shown in FIG. 1, the system 100includes a paper machine 102, a controller 104, and a network 106. Thepaper machine 102 includes various components used to produce a paperproduct, namely a paper web 108 that is collected at a reel 110. Thecontroller 104 monitors and controls the operation of the paper machine102, which may help to maintain or increase the quality of the paper web108 produced by the paper machine 102. In the following description, themachine direction (MD) of the web 108 denotes the direction along the(longer) length of the web 108.

In this example, the paper machine 102 includes at least one headbox112, which distributes a pulp suspension uniformly across the machineonto a continuous moving wire screen or mesh 113. The pulp suspensionentering the headbox 112 may contain, for example, 0.2-3% wood fibers,fillers, and/or other materials, with the remainder of the suspensionbeing water.

Arrays of drainage elements 114, such as vacuum boxes, remove as muchwater as possible to initiate the formation of the web 108. An array ofsteam actuators 116 produces hot steam that penetrates the paper web 108and releases the latent heat of the steam into the paper web 108. Anarray of rewet shower actuators 118 adds small droplets of water (whichmay be air atomized) onto the surface of the paper web 108. The paperweb 108 is then often passed through a calender having several nips ofcounter-rotating rolls. Arrays of induction heating actuators 120 heatthe shell surfaces of various ones of these rolls.

Two additional actuators 122-124 are shown in FIG. 1. A thick stock flowactuator 122 controls the consistency of incoming stock received at theheadbox 112. A steam flow actuator 124 controls the amount of heattransferred to the paper web 108 from drying cylinders. The actuators122-124 could, for example, represent valves controlling the flow ofstock and steam, respectively. These actuators may be used forcontrolling the dry weight and moisture of the paper web 108. Additionalflow actuators may be used to control the proportions of different typesof pulp and filler material in the thick stock and to control theamounts of various additives (such as retention aid or dyes) that aremixed into the stock.

This represents a brief description of one type of paper machine 102that may be used to produce a paper product. Additional detailsregarding this type of paper machine 102 are well-known in the art andare not needed for an understanding of this disclosure.

In order to control the paper-making process, one or more properties ofthe paper web 108 may be continuously or repeatedly measured. The webproperties can be measured at one or various stages in the manufacturingprocess. This information may then be used to adjust the paper machine102, such as by adjusting various actuators within the paper machine102. This may help to compensate for any variations of the webproperties from desired targets, which may help to ensure the quality ofthe web 108.

As shown in FIG. 1, the paper machine 102 includes one or more scanners126-128, each of which may include one or more sensors. Each scanner126-128 is capable of measuring one or more characteristics of the paperweb 108. For example, each scanner 126-128 could include sensors formeasuring the tension, caliper, moisture, anisotropy, basis weight,color, gloss, sheen, haze, surface features (such as roughness,topography, or orientation distributions of surface features), or anyother or additional characteristics of the paper web 108.

Each scanner 126-128 includes any suitable structure or structures formeasuring or detecting one or more characteristics of the paper web 108,such as one or more sets of sensors. The use of scanners represents oneparticular embodiment for measuring web properties. Other embodimentscould be used, such as those including one or more stationary sets orarrays of sensors, deployed in one or a few locations across the web ordeployed in a plurality of locations across the whole width of the websuch that substantially the entire web width is measured.

The controller 104 receives measurement data from the scanners 126-128and uses the data to control the paper machine 102. For example, thecontroller 104 may use the measurement data to adjust any of theactuators or other components of the paper machine 102. The controller104 includes any suitable structure for controlling the operation of atleast part of the paper machine 102, such as a computing device. Notethat while a single controller 104 is shown here, multiple controllers104 could be used, such as different controllers that control differentvariables of the web.

The network 106 is coupled to the controller 104 and various componentsof the paper machine 102 (such as the actuators and scanners). Thenetwork 106 facilitates communication between components of the system100. The network 106 represents any suitable network or combination ofnetworks facilitating communication between components in the system100. The network 106 could, for example, represent a wired or wirelessEthernet network, an electrical signal network (such as a HART orFOUNDATION FIELDBUS network), a pneumatic control signal network, or anyother or additional network(s).

The controller(s) 104 can operate to control one or more aspects of thepaper machine 102 using one or more models. For example, each modelcould associate one or more manipulated variables with one or morecontrolled variables. A controlled variable generally represents avariable that can be measured or inferred and that is ideally controlledto be at or near a desired setpoint or within a desired range of values.A manipulated variable generally represents a variable that can beadjusted in order to alter one or more controlled variables.

In order to tune a controller 104, at least one operator console 130 cancommunicate with the controller 104 over a network 132. The operatorconsole 130 generally represents a computing device that supports one ormore techniques for robust tuning of MPC and other model-basedcontrollers that are used to control MIMO processes having modeluncertainty. The techniques for robust tuning of model-based controllersare described in more detail below. The techniques for robust tuninggenerally involve identifying tuning parameters for a controller andoutputting the tuning parameters to the controller for use duringsubsequent control operations. The network 132 represents any suitablenetwork or combination of networks that can transport information, suchas an Ethernet network.

In this example, the operator console 130 includes one or moreprocessing devices 134, one or more memories 136, and one or moreinterfaces 138. Each processing device 134 includes any suitableprocessing or computing device, such as a microprocessor,microcontroller, digital signal processor, field programmable gatearray, application specific integrated circuit, or discrete logicdevices. Each memory 136 includes any suitable storage and retrievaldevice, such as a random access memory (RAM) or Flash or other read-onlymemory (ROM). Each interface 138 includes any suitable structurefacilitating communication over a connection or network, such as a wiredinterface (like an Ethernet interface) or a wireless interface (like aradio frequency transceiver).

Note that while the operator console 130 is described as implementingthe technique(s) for robust tuning of model-based controllers, othertypes of devices could also be used. For instance, the operator console130 could interact with a server 140, and the server 140 could actuallyexecute the algorithms used to implement one or more techniques forrobust tuning of model-based controllers. In this case, the operatorconsole 130 could present a graphical user interface and interact with auser. The server 140 could include one or more processing devices, oneor more memories, and one or more interfaces (similar to the operatorconsole 130).

Although FIG. 1 illustrates one example of a web manufacturing orprocessing system 100, various changes may be made to FIG. 1. Forexample, other systems could be used to produce other paper or non-paperproducts. Also, while shown as including a single paper machine 102 withvarious components and a single controller 104, the system 100 couldinclude any number of paper machines or other machinery having anysuitable structure, and the system 100 could include any number ofcontrollers. In addition, FIG. 1 illustrates one example operationalenvironment in which MPC or other model-based controller(s) can betuned. This functionality could be used in any other suitable system,regardless of whether the system is used to manufacture or process websof material.

In the following description, robust tuning techniques are describedwith respect to a tuning problem for an MD-MPC controller in a MIMOprocess under parametric uncertainty, where the techniques considertime-domain performance indices. “MD-MPC” here indicates that an MPCcontroller being tuned is used to control an MD property of a web 108.

The parameter tuning problem for an MPC or other model-based controlleris often challenging, and it becomes even more challenging when MIMOprocesses are considered. One conventional approach for parameter tuningof an MPC controller is to fix all MPC tuning parameters except one inorder to identify a tuning problem with one degree of freedom (1DOF). Byadjusting this parameter, an analysis of the relationship between theclosed-loop performance of the controller and the corresponding degreeof freedom can occur, and some tuning guidelines can be developed basedon this type of analysis. In another conventional approach, MPCparameters for a MIMO system can be tuned by solving two semi-definiteprogramming problems sequentially in order to achieve a desiredclosed-loop performance. In yet another conventional approach,controller parameters are tuned by matching an unconstrained MIMO MPCwith a preassigned multivariable controller so that the properties ofthe preassigned controller could be inherited. Still other conventionalapproaches have involved a tuning-friendly MPC framework in which atuning parameter is used to adjust a controller's performance. However,despite these approaches, a user-friendly tuning approach for uncertainMIMO processes is still missing.

U.S. patent application Ser. No. 13/907,495 (hereby incorporated byreference in its entirety) discloses a two degree of freedom (2DOF) MPCtuning framework. U.S. patent application Ser. No. 14/314,221(incorporated by reference above) uses this framework to, among otherthings, provide for robust tuning of a single-input, single-output(SISO) system. This disclosure uses the 2DOF MPC tuning framework tohelp simplify the tuning of MPC or other model-based controllers forMIMO systems with some model uncertainty associated with the MIMOsystems.

System Description and Problem Formulation

FIG. 2 illustrates an example internal model control structure 200employed for MPC of an uncertain MIMO process according to thisdisclosure. This closed-loop system includes three main parts: a processmodel (G₀) 202, an MPC controller 204, and user-specified filters (F_(r)and F_(d)) 206-208. The process model 202 is used to mathematicallyrepresent a multivariable MIMO process (G₀) 210. There is also typicallysome time delay 212 associated with feedback in the closed-loop system,which is shown here as a first-order delay but could include delay(s) ofother or additional order(s).

In FIG. 2,

and

are transfer function matrices of the real industrial process and thenominal process model, respectively. In some embodiments, all subsystemsin G_(p) and G₀ have a first-order-plus-dead-time (FOPDT) structure.Therefore, the real process G_(p) 210 can be represented as:

$\begin{matrix}{{{G_{p}(s)} = \begin{bmatrix}{G_{p}^{11}(s)} & \ldots & {G_{p}^{1n}(s)} \\\vdots & \ddots & \vdots \\{G_{p}^{m\; 1}(s)} & \ldots & {G_{p}^{mn}(s)}\end{bmatrix}},{{G_{p}^{ij}(s)} = {\frac{k_{ij}}{{\tau_{ij}s} + 1}e^{{- T_{ij}}s}}},{1 \leq i \leq m},{1 \leq j \leq n},} & (1)\end{matrix}$where k_(ij), τ_(ij), and T_(ij) are the real process gain, timeconstant, and time delay for G_(p) ^(ij), respectively, and ij denotesthe pair of i^(th) input and j^(th) output of the MIMO process. As modelmismatches, disturbances, and noise exist in practice, G_(p)(s) istypically not known with exact certainty. Thus, a nominal model G₀(s)can be identified as an approximation of G_(p)(s). For example, thenominal model G₀(s) could be expressed as:

$\begin{matrix}{{{G_{0}(s)} = \begin{bmatrix}{G_{0}^{11}(s)} & \ldots & {G_{0}^{1n}(s)} \\\vdots & \ddots & \vdots \\{G_{0}^{m\; 1}(s)} & \ldots & {G_{0}^{mn}(s)}\end{bmatrix}},{G_{0}^{ij} = {\frac{k_{ij}^{0}}{{\tau_{ij}^{0}s} + 1}e^{{- T_{ij}^{0}}s}}},{1 \leq i \leq m},{1 \leq j \leq n},} & (2)\end{matrix}$where k_(ij) ⁰, τ_(ij) ⁰, and T_(ij) ⁰ can be obtained from a processidentification software tool based on input/output data of the realprocess or in any other suitable manner.

Because differences between the actual process 210 and the identifiednominal model 202 are typically inevitable, this model mismatch can betaken into consideration when performing controller tuning. Two types ofuncertainties are routinely used in robust control, namely parametricuncertainty and unstructured uncertainty. Unstructured uncertainty isoften difficult to understand and specify by end users. Therefore, insome embodiments, only parametric uncertainty may be considered forparameter tuning during robust tuning of MPC and other model-basedcontrollers used with MIMO processes having model uncertainty. Note,however, that other approaches could explicitly consider unstructureduncertainty.

Parametric uncertainty can be modeled in the following form:k _(ij) ϵ[k _(ij) ,k _(ij)],τ_(ij)ϵ[τ _(ij),τ _(ij) ],T _(ij) ϵ[T _(ij),T _(ij)],1≤i≤m,1≤j≤n,  (3)where k _(ij) and k _(ij), τ _(ij) and τ _(ij), and T _(ij) and T _(ij)are the lower and upper bounds defining ranges or intervals of mismatchfor the three model parameters k_(ij), τ_(ij), and T_(ij). Based onthis, a set of possible perturbed process models can be denoted as:Π:={G _(p)(s):k _(ij) ϵ[k _(ij) ,k _(ij)],τ_(ij)ϵ[τ _(ij),τ _(ij) ],T_(ij) ϵ[T _(ij) ,T _(ij)],1≤i≤m,1≤j≤n}.  (4)

With respect to the MPC controller 204, let H_(u) denote the controlhorizon, and let H_(p) denote the prediction horizon (where1≤H_(u)≤H_(p)). In some embodiments, the MPC controller 204 in FIG. 2operates to solve the following finite-horizon optimal control problem:

$\begin{matrix}{{{{{\min\limits_{\Delta\; U}\; J} = {{( {{\hat{Y}(k)} - {Y_{ref}(k)}} )^{T}{Q_{1}( {{\hat{Y}(k)} - {Y_{ref}(k)}} )}} + {\Delta\;{U(k)}^{T}Q_{2}\Delta\;{U(k)}} + {( {{U_{H_{u}}(k)} - {U_{ref}(k)}} )^{T}{Q_{3}( {{U_{H_{u}}(k)} - {U_{ref}(k)}} )}}}}\mspace{20mu}{s.t.\mspace{14mu}{\hat{x}( {k + i} )}}} = {{A^{i}{\hat{x}(k)}} + {\sum\limits_{j = 1}^{\min{\{ H_{u,i}\}}}{A^{i - j}B\;\Delta\;{u( {k + j - 1} )}}}}},\mspace{79mu}{{\hat{y}( {k + i} )} = {C{\hat{x}( {k + i} )}}},{{{for}\mspace{14mu} i} = 1},2,\ldots\mspace{14mu},H_{p},} & (5)\end{matrix}$where:

$\begin{matrix}{{{\hat{Y}(k)} = \begin{bmatrix}{\hat{y}( {k + 1} )} \\{\hat{y}( {k + 2} )} \\\vdots \\{\hat{y}( {k + H_{p}} )}\end{bmatrix}},{{\Delta\;{U(k)}} = \begin{bmatrix}{\Delta\;{u(k)}} \\{\Delta\;{u( {k + 1} )}} \\\vdots \\{\Delta\;{u( {k + H_{u} - 1} )}}\end{bmatrix}},} & (6) \\{{U_{H_{u}}(k)} = {{\begin{bmatrix}1 \\1 \\\vdots \\1\end{bmatrix}{u( {k - 1} )}} + {\begin{bmatrix}1 & 0 & \ldots & 0 \\1 & 1 & \ddots & \vdots \\\vdots & \ddots & \ddots & 0 \\1 & \ldots & 1 & 1\end{bmatrix}\Delta\;{{U(k)}.}}}} & (7)\end{matrix}$Here, U_(ref) and Y_(ref) are reference signal vectors of U_(H) _(u) andŶ, respectively, and Q₁, Q₂, and Q₃ are weighting matrices.

Given the MPC controller 204 and process model 202 described above, thetwo user-specified filters F_(r) and F_(d) 206-208 in FIG. 2 are usedrespectively for filtering an output target y_(tgt)(k) and an estimateddisturbance ({circumflex over (d)}

y(k)−ŷ(k)), which constitute the MIMO 2DOF MPC structure. Based on thefiltered signals, the reference trajectory Y_(ref)(k) can be calculatedas follows:

$\begin{matrix}{{{Y_{ref}(k)} = {\begin{bmatrix}{y_{ref}( {k + 1} )} \\{y_{ref}( {k + 2} )} \\\vdots \\{y_{ref}( {k + H_{p}} )}\end{bmatrix} = {{F_{r}( {y_{tgt}(k)} )} - {F_{d}( {\hat{d}(k)} )}}}},} & (8)\end{matrix}$where F_(r)(⋅) and F_(d)(⋅) are projection filters generated accordingto f_(r)(z) and f_(d)(z), and wherey_(ref)(z)=f_(r)(z)y_(tgt)(z)−f_(d)(z){circumflex over (d)}(z). Here,f_(r)(z) and f_(d)(z) denote a reference tracking filter and adisturbance rejecting filter, which in some embodiments can be definedas follows:

$\begin{matrix}{{{{f_{r}(z)} = \begin{bmatrix}{f_{r,1}(z)} & 0 & 0 \\0 & \ddots & 0 \\0 & 0 & {f_{r,m}(z)}\end{bmatrix}},{and}}{{{f_{r,i}(z)}:={\frac{b_{r,i}z^{- 1}}{1 - {a_{r,i}z^{- 1}}}z^{- T_{i}}}},{1 \leq i \leq m},}} & (9)\end{matrix}$where

${a_{r,i} = e^{- \frac{Ts}{\tau_{i}\lambda_{i}}}},$b_(r,i)=1−a_(r,i); f_(r,i)(z) is the reference tracking filter for thei^(th) output. Define λ=[λ₁, λ₂, . . . , λ_(m)] as the referencetracking performance ratio vector, and define f_(d)(z) as having thefollowing form:

$\begin{matrix}{{{{f_{d}(z)} = \begin{bmatrix}{f_{d,1}(z)} & 0 & 0 \\0 & \ddots & 0 \\0 & 0 & {f_{d,m}(z)}\end{bmatrix}},{and}}{{{f_{d,i}(z)}:={\frac{b_{d,i}z^{- 1}}{1 - {a_{d,i}z^{- 1}}}z^{- T_{i}}}},{1 \leq i \leq m},}} & (10)\end{matrix}$where

$a_{d,i} = e^{{- \frac{Ts}{\tau_{i}\lambda_{d,i}}},}$b_(d,i)=1−a_(d,i), and f_(d,i)(z) is the disturbance rejection filterfor the i^(th) output. Similarly, define λ_(d)=[λ_(d,1), λ_(d,2), . . ., λ_(d,m)] as the disturbance rejection performance ratio vector. Areference tracking performance ratio defines a ratio between a desiredclosed-loop reference tracking response time of a controller and anopen-loop response time of an industrial process. A disturbancerejection performance ratio defines a ratio between a desiredclosed-loop disturbance rejecting response time of a controller and theopen-loop response time of the industrial process. In this way, the MPCcontroller's performance can be adjusted by tuning λ and λ_(d) withfixed penalty matrices (which in some embodiments could be defined asQ₁=I; Q₂=0.01×I; and Q₃=0, although other values could be used). As aresult, the MPC tuning problem can be simplified.

As can be seen here, one part of the 2DOF MPC tuning problem is thedesign of f_(r,i)(z) and f_(d,i)(z) in Equations (9) and (10). In someembodiments, the design of f_(r,i)(z) is based on, given the i^(th)output, the open-loop transfer function from the input that dominatesthe output that is selected to construct this filter. One reason forthis is to make the speed of the closed-loop response dependent on thespeed of the dominant or primary open-loop response of the systemaccording to the requirements for a specific product being produced. Asfor f_(d,i)(z), the same design procedure can be applied, except thatλ_(d,i) can be used instead of λ_(i) such that the output target and theestimated disturbance can be filtered separately.

Based on this, λ and λ_(d) (which may be referred to as theλ-parameters) are vectors with appropriate dimensions and are utilizedto tune the closed-loop performance of the control structure 200,instead of (or in addition to) tuning the weighting matrices in an MPCcost function. Performance indices are specified via values such asovershoot, total variation, and settling time for each output of theMIMO process. This maintains the user-friendliness of the proposedtuning approach. However, due to inevitable time delays in processoperations and model mismatches in model identification, the expressionsof the closed-loop responses typically cannot be obtained explicitly,and the tuning problem is therefore difficult to formulate. Moreover,the complexity of the tuning problem can increase based on the system'ssize, and the relationship between the λ-parameters and each output ofthe MIMO system can be unclear. In addition, to guarantee a satisfactoryuser experience, the computation time of the tuning procedure should notonly be limited by a specified amount of time but also be predictablewithout running the algorithm.

In accordance with this disclosure, the operator console 130, server140, or other component(s) support an efficient heuristic approach tofind a solution to the tuning problem for an MPC or other model-basedcontroller used with an uncertain MIMO process. This approach supportsthe following capabilities.

-   -   An efficient time-domain performance visualization technique for        MIMO processes is proposed, based on which the characterizations        of possible step responses of a set of systems described by        parametric uncertainties are obtained.    -   The tuning problem is cast into a constrained optimization        problem and then transformed into a number of individual        multiple-input, single-output (MISO) tuning problems, based on        which the tuning problem is simplified. Utilizing the        visualization technique and the simplified tuning problem, a        fast tuning algorithm is disclosed, and controller parameters        can be tuned for satisfactory performance within acceptable        computation times.    -   An efficient technique to predict the overall computation time        of the proposed tuning algorithm is disclosed, based on which        end users can predict the tuning time without running the        algorithm.

For a MIMO system, the overshoot, total variation, and settling time ofeach output can be considered as tuning measurements. Since theparametric uncertainty defined in Equation (3) results in a set ofperturbed systems, the worst-case time-domain performance indices can beemployed. Definitions for the worst-case overshoot, settling time, andtotal variation are as follows. The worst-case overshoot OS of a set ofstep responses with the same final value is the maximum value in allresponses minus the final value divided by the final value. Theworst-case settling time T_(s) of a set of step responses with the samefinal value is the minimum time required for all responses to reach andstay within a range of a pre-specified percentage of the final value.The worst-case total variation TV, assuming the system converges to atarget value within n steps, is:

$\begin{matrix}{{TV} = {\max\limits_{G_{p} \in \prod}{\sum\limits_{k = 1}^{n}{{{{y(k)} - {y( {k - 1} )}}}.}}}} & (11)\end{matrix}$These definitions are provided to characterize the worst-caseperformance for a set of step responses for SISO systems and thus areapplicable for each of the outputs of a MIMO system. In someembodiments, end users can specify their requirements on each outputdirectly via the worst-case time-domain performance indices, whichimproves the user-friendliness of the tuning algorithm.

Moreover, the tuning time (denoted t_(λ)) can be an important factor ina successful industrial process controller tuner design. For example,according to some guidelines of commercial paper machine tuner designs,the total time consumption of a tuning algorithm should not be more thanhalf a minute, and in some embodiments this requirement can beconsidered as a hard constraint for the tuning algorithm disclosed inthis patent document. Note that tuning time is normally not consideredin existing MIMO tuning algorithms because (i) the limited computationtime might affect the feasibility of these algorithms and (ii) thecomputation time is normally not predictable. A hard constraint on thetuning time can further increase the difficulty of solving the MIMOtuning problem.

Based on the factors mentioned above, some objectives of the tuningalgorithm disclosed here include (i) determining λ and λ_(d) so that aclosed-loop system (such as that shown in FIG. 2) is robustly stable andall outputs track their targets with fast responses, small overshoots,and small total variations, (ii) doing so within an acceptable tuningtime, and (iii) doing so under the parametric uncertainties defined inEquation (3). In some embodiments, to accomplish these objectives, thetuning problem can be formulated as:

$\begin{matrix}{{\min\limits_{\lambda,\lambda_{d}}{{T_{s}( {\lambda,\lambda_{d}} )}}_{\infty}},{{s.t.\mspace{14mu}{{OS}_{i}( {\lambda,\lambda_{d}} )}} \leq {OS}_{i}^{*}},{1 \leq i \leq m},{{{TV}_{i}( {\lambda,\lambda_{d}} )} \leq {{TV}_{i}^{*}\mspace{14mu} 1} \leq i \leq m},{t_{\lambda} \leq {30\mspace{14mu} s}},} & (12)\end{matrix}$where λ=[λ₁, λ₂, . . . , λ_(m)] and λ_(d)=[λ_(d,1), λ_(d,2), . . . ,λ_(d,m)]. Here, OS(λ, λ_(d)):=[OS₁(λ, λ_(d)), OS₂(λ, λ_(d)), . . . ,OS_(m)(λ, λ_(d))], TV(λ, λ_(d)):=[TV₁(λ, λ_(d)), TV₂(λ, λ_(d)), . . . ,TV_(m)(λ, λ_(d))], and T_(s)(λ, λ_(d)):=[T_(s,1)(λ, λ_(d)), T_(s,2)(λ,λ_(d)), . . . , T_(s,m)(λ, λ_(d))] denote the worst-case overshoots,total variations, and settling times given (λ, λ_(d)) for all outputs ofthe MIMO system G_(p)(s) within the parametric uncertainty region,meaning G_(p)(s)ϵΠ. Also, OS*:=[OS₁*, OS₂*, . . . , OS_(m)*] andTV*:=[TV₁*, TV₂*, . . . , TV_(m)*] denote the specifications on theovershoots and total variations. Note that in Equation (12), λ and λ_(d)are tuned by minimizing the worst-case settling times of all outputswith the corresponding worst-case OS and TV lying within certaintolerable regions. As noted above, in some embodiments, the tuning timemay be no more than half a minute (t_(λ)≤30 s).

Envelope Algorithm: MIMO Systems

This section describes a performance visualization technique to obtainthe worst-case overshoots, settling times, and total variations for theoutputs of a MIMO system. One goal here is to graphically characterizethe envelopes of the responses of a set of MIMO systems satisfyingG_(p)(s)ϵΠ given the values of λ and λ_(d). As the system consideredhere has multiple outputs, an envelope is represented for each output.This allows end users to easily determine the effect of choosing acombination of λ and λ_(d).

In U.S. patent application Ser. No. 14/314,221 (incorporated byreference above), a visualization technique was described thatcharacterizes the envelopes of the responses of a set of SISO FOPDTsystems G_(p)*(s) under parametric uncertaintyG_(p)*(s)ϵΠ_(e):={G_(p)*(s):kϵ[k, k], τϵ[τ, τ], Tϵ[T, T]} via eightextreme case systems: Π_(E):={G_(p)*(s):kϵ{k, k}, τϵ{τ, τ}, Tϵ{T, T}},where k, τ, and T are the gain, time constant, and time delay,respectively. Although all subsystems in the MIMO process G_(p)(s) havea FOPDT structure and the parametric uncertainty of G_(p)(s) isconstructed by considering all subsystems in a way like Π_(e), theapproach mentioned above may not be used directly with a MIMO processdue to the following factors. First, the number of extreme-case systemsdepends on the number of model parameters and becomes a large numbereven for a low-dimensional MIMO system (for instance, a 2-by-2 systemincludes four FOPDT subsystems and thus requires 84 extreme-casesystems). Second, the computation time of the visualization method inthe approach mentioned above increases rapidly as the number ofextreme-case systems increases. Third, as the tuning procedure should beshort (such as less than 30 seconds) and the performance visualizationis only a small step in the overall tuning procedure, the computationtime allowed for obtaining the envelopes may be very limited.

In some embodiments, to deal with this problem, the parameters of a MIMOsystem can be defined as follows:

$\begin{matrix}{{{k = \begin{bmatrix}k_{11} & \ldots & k_{1n} \\\vdots & \ddots & \vdots \\k_{m\; 1} & \ldots & k_{mn}\end{bmatrix}},{\tau = \begin{bmatrix}\tau_{11} & \ldots & \tau_{1n} \\\vdots & \ddots & \vdots \\\tau_{m\; 1} & \ldots & \tau_{mn}\end{bmatrix}}}T = {\begin{bmatrix}T_{11} & \ldots & T_{1n} \\\vdots & \ddots & \vdots \\T_{m\; 1} & \ldots & T_{mn}\end{bmatrix}.}} & (13)\end{matrix}$Based on this, the MIMO system with the parametric uncertainty asdefined in Equation (3) can be approximated as:

_(E) :={G _(p)(s):kϵ{k,k },τϵ{τ,τ},Tϵ{T,T}},  (14)where k, τ and T denote all entries in k, τ, and T taking their minimumvalues in the uncertainty region and k, τ, and T denote all entries ink, τ, and T taking their maximum values in the uncertainty region. Notethat

_(E) only includes eight extreme-case systems. Therefore, the envelopesof the MIMO system with the parametric uncertainty as defined inEquation (3) can be obtained via

_(E) with significantly reduced the computation time.

In some embodiments, based on

_(E), the envelopes of the step responses of a MIMO system with an MPCor other model-based controller can be obtained using the algorithmshown in Table I below. Although this approach may not be guaranteed tobe optimal, it is intuitive in that the extreme behavior of the stepresponses mostly happens when all extreme process parameters are reachedsimultaneously. The effectiveness of this algorithm has been verifiedvia extensive simulations.

TABLE I Algorithm 1 MIMO envelope algorithm 1: Input λ_(d), λ and theuncertainty intervals [k, k], [τ, τ] and [T, T]; 2: Calculate all theoutput responses Y_(i) = [y₁, . . . , y_(m)]_(i) ^(T) based on the ithextreme combinations of the uncertainty intervals (i = 1, 2, . . . , 8)in  

 _(E). 3: Obtain the upper envelope for all the outputs {y ₁, . . . , y_(m)} by calculating max_(i∈{1, 2, . . . , 8}) Y_(i)(k,:), k = 1, 2, . .. , m. 4: Obtain the lower envelope for all the outputs {y ₁, . . . , y_(m)} by calculating min_(i∈{1, 2, . . . , 8}) Y_(i)(k,:), k = 1, 2, . .. , m.

MIMO Tuning Algorithm

This section discusses the actual tuning algorithm for model-basedcontrollers of uncertain MIMO processes. As explained below, the tuningalgorithm represents an iterative tuning procedure that determines thevalues of λ and λ_(d) based on the visualization technique described inthe prior section. In order to solve Equation (12), the relationshipbetween the λ-parameters and the performance indices of each output,such as OS_(i)(λ, λ_(d)) and T_(s,i)(λ, λ_(d)) in Equation (12), isexplored. As the analytical expressions of the performance indices maynot exist, an empirical analysis can be carried out to aid the tuningprocedure.

According to the 2DOF tuning structure, the closed-loop system'sbehavior is governed by the F_(r) and F_(d) filters 206-208. As bothfilters 206-208 have diagonal structures and λ and λ_(d) only exist onthe diagonal entries, it is intuitive to assume that the closed-loopresponse of the i^(th) output is dominated by the i^(th) elements in λand λ_(d) (which are denoted λ_(i) and λ_(d,i)). Also, given each outputof the MIMO system, the corresponding F_(r) and F_(d) filters 206-208can be designed based on the dominant or primary open-loop systemproducing that output. This also helps to ensure that the specific pairof λ-parameters plays a dominant role only for the closed-loop responseof the single corresponding output. In this way, OS_(i)(λ, λ_(d)) andT_(s,i)(λ, λ_(d)) are functions of only λ_(i) and λ_(d,i). In otherwords, OS_(i)(λ, λ_(d))=OS_(i)(λ_(i), λ_(d,i)) and T_(s,i)(λ,λ_(d))=T_(s,i)(λ_(i), λ_(d,i)).

Examples of these properties are illustrated in FIGS. 3A and 3B via atypical 2-by-3 MD process. In particular, FIG. 3A illustrates anumerical verification of OS_(i)(λ, λ_(d)), and FIG. 3B illustrates anumerical verification of T_(s,i)(λ, λ_(d)). These illustrations havebeen verified through extensive simulations with other types of MIMOsystems. Based on FIGS. 3A and 3B, two observations can be made. First,from the right graphs in FIGS. 3A and 3B, it can be seen that theadjustment of λ₁ and λ_(d,1) does not affect the overshoot and settlingtime of the second output significantly if at all, which verifies theidea above. Second, the overshoot could be empirically treated as amonotonic function of λ, and the settling time could be considered as aunimodal function of λ_(d), which can basically be determined by the2DOF MPC structure.

Thus, as the closed-loop response of the i^(th) output is dominated byλ_(i) and λ_(d,i), the tuning problem in Equation (12) can be simplifiedfrom a MIMO tuning problem to m individual MISO tuning problems in whichλ_(i) and λ_(d,i) are tuned for the i^(th) output separately.Mathematically, the new MISO tuning problem can be expressed as:

$\begin{matrix}{{\min\limits_{\lambda_{i},\lambda_{d,i}}{T_{s,i}( {\lambda_{i},\lambda_{d,i}} )}},{{s.t.\mspace{14mu}{{OS}_{i}( {\lambda_{i},\lambda_{d,i}} )}} \leq {{OS}_{i}^{*}\mspace{11mu} 1} \leq i \leq m},{{{TV}_{i}( {\lambda_{i},\lambda_{d,i}} )} \leq {{TV}_{i}^{*}\mspace{14mu} 1} \leq i \leq m},{t_{\lambda} \leq {30\mspace{14mu} s}},} & (15)\end{matrix}$based on which the tuning algorithm described below is proposed.

It is worth noting that end users are often not familiar with “totalvariation.” Various approaches could therefore be used to handle thisissue, such as by specifying the total variation TV_(i)* based on theovershoot according to TV_(i)*=1+2 OS_(i)* or based on the decay ratio(DR*) according to TV_(i)*=1+[3OS_(i)*/(1−DR_(i)*)]. The overshoot-basedTV_(i)* is slightly conservative, while the decay ratio-based TV_(i)*relies on additional knowledge of the decay ratio for the end users.Thus, users can choose either option in different situations dependingon their preferences and process knowledge.

Based on the empirical properties of OS_(i)(λ, λ_(d)) and T_(s,i)(λ,λ_(d)), an efficient MIMO tuning algorithm is now described. AsT_(s,i)(λ, λ_(d)) is tuned using only λ_(i) and λ_(d,i) in the MISOtuning problem of Equation (15), it is similar to the SISO tuningproblem described in U.S. patent application Ser. No. 14/314,221. Onefeasible method to the tuning problem is to solve each of the MISOtuning problems one-by-one via the proposed counter-line searchingtuning algorithm in U.S. patent application Ser. No. 14/314,221.However, this method may be too time-consuming in certain situationswhere there are constraints on tuning time. Based on Algorithm 1 above,instead of solving the MISO tuning problems one after the other, a MIMOtuning algorithm can solve all MISO tuning problems simultaneously bysearching for the optimal λ-parameters for all of the MISO systems alongm identical counter-lines at the same time. Because of this, theproposed tuning algorithm is almost as fast as solving only one MISOtuning problem. The fast algorithm is feasible because the counter-linesearching is based on the worst-case performance indices calculated byAlgorithm 1 and, when Algorithm 1 is executed, the performance indicesare computed for all of the MISO systems. Moreover, as all the MISOtuning problems are almost independent (tuning of (λ_(i), λ_(d,i)) doesnot affect that of (λ_(j), λ_(d,j)) where j≠i), the proposed tuningalgorithm below can produce the same results as those of tuning the MISOsystems one after another.

To implement this approach, the MIMO tuning algorithm described belowincludes two sub-algorithms. The first sub-algorithm (Algorithm 2 inTable II) finds the corresponding λ_(i), i=1, . . . , m on a contourline for a fixed value of λ_(d,i), i=1, . . . , m (denoted λ_(i) ^(c))for all of the MISO systems. The second algorithm (Algorithm 3 in TableIII) finds the optimal λ_(d,i), i=1, . . . , m that leads to thesmallest values of settling times for all of the outputs based onAlgorithm 2.

TABLE II Algorithm 2 Find λ^(c)(TV*, λ_(d)) and T_(s) ^(c)(TV*, λ_(d)). 1: Input TV*, λ_(d), and the uncertainty intervals [k, k], [τ, τ] and[T, T] and upper bound for λ: λ*;  2: Input ∈;  3: NoFeasibleLambda =1_(1×m);  4: λ ← 0.5_(1×m), λ ← λ*;  5: while max (λ − λ) > ∈ do  6: for i=1 : m do  7:   λ(i) ← (λ(i) + λ(i)) × 0.5  8:  end for  9: Numerically evaluate the total variation and overshoot vectors TV(λ,λ_(d)) and OS(λ, λ_(d)) 10:  for i=1 : m do 11:   if TV(i) > TV_(i)*then 12:    λ(i) ← λ(i); 13:   else 14:    if OS(i) < OS_(i)* then 15:    λ(i) ← λ(i); 16:     NoFeasibleLambda(i) = 0; 17:    else 18:    λ(i) ← λ(i); 19:    end if 20:   end if 21:  end for 22: end while23: λ^(c)(TV*, λ_(d)) ← (λ + λ)/2 ; 24: T_(s) ^(c)(TV*, λ_(d)) ←T_(s)(λ^(c), λ_(d)); 25: end

TABLE III Algorithm 3 Tuning of λ and λ_(d). 1: Input uncertaintyintervals [k, k], [τ, τ] and [T, T] and upper bound for λ_(d): λ*_(d);2: Input the overshoot specification OS*; 3: Calculate the totalvariation specification according to${{TV}^{*} = {{1 + {2{OS}^{*}\mspace{14mu}{or}\mspace{14mu}{TV}^{*}}} = {1 + \frac{3{OS}^{*}}{1 - {DR}^{*}}}}};$4: Input ϵ; 5: λ _(d) ← 0.5_(1×m), λ _(d) ← λ*_(d); 6: while max (λ _(d)− λ _(d)) > ϵ do 7:  for i = 1 : m do 8:   λ_(d1)(i) ← λ _(d)(i) + (λ_(d)(i) − λ _(d)(i)) × 0.382; 9:   λ_(d2)(i) ← λ _(d)(i) + (λ _(d)(i) −λ _(d)(i)) × 0.618; 10:  end for 11:  Numerically evaluate the settlingtimes T^(c) _(s1)(TV*, λ_(d1)) and T^(c) _(s2)(TV*, λ_(d2)); 12:  for i= 1 : m do 13:   if NoFeasibleLambda(i) = 0 then 14:    if T^(c)_(s1)(i) > T^(c) _(s2)(i) then 15:     λ _(d)(i) ← λ_(d1)(i); 16:   else 17:     λ _(d)(i) ← λ_(d2)(i); 18:    end if 19:   else 20:    λ_(d)(i) ← λ_(d1)(i); 21:   end if 22:  end for 23: end while 24: λ_(d) ←(λ _(d) + λ _(d)) / 2; 25: λ ← λ^(c)(TV*, λ_(d)); 26: end

In Algorithm 2, since TV_(i)(λ_(i), λ_(d,i)) is a monotonic function ofλ_(i) for a fixed λ_(d,i), the search of λ_(i) ^(c) that leads to anygiven value of TV_(i)* for a fixed λ_(d,i) can be achieved by abisection search. T_(s) ^(c)(TV*, λ_(d)) is used to represent thedependence of the worst-case settling time on TV* and λ_(d) to avoidnotational confusion. Note that all of the λ_(i) values can be tunedbefore evaluating the TV and OS for all of the outputs, based on whichthe search on all of the contour-lines are executed simultaneouslyrather than one after another. Also, note that lines 14-19 of Algorithm2 are used if TV* is chosen based on the decay ratio technique.

In Algorithm 3, for each MISO system, the worst-case settling time canbe approximately a unimodal function of λ_(d,i). The underlyingassumption is that λ_(d,i) controls the robust stability, so a smallerλ_(d,i) leads to an aggressive and oscillatory response while a largerλ_(d,i) leads to a sluggish response. Thus, a golden search could beutilized in Algorithm 3 to find the optimal λ_(d,i), i=1, . . . , m thatoffers the smallest worst-case settling times for all of the MISOsystems. The corresponding value of λ_(i), i=1, . . . , m can beobtained through Algorithm 2.

The fast tuning algorithm described above not only solves the MIMOtuning problem efficiently, it also provides the ability to accuratelypredict the tuning time. The ability to accurately predict the tuningtime is highly demanded or desired with algorithms designed forcommercial tuner software because the user-friendliness can be improved.In this case, since the proposed tuning algorithm is a line search-basedmethod, the computation time can be estimated based on the number ofiterations needed for convergence and the time required for eachiteration. The number of iterations may only depend on the predefinedsearch region and stop criteria, and the iteration time can be decidedby the time required for numerically evaluating the performance indices(the duration of which can be computed based on the time required tocall Algorithm 1, which can also be pre-calculated). Thus, the tuningtime can be predicted with Algorithm 4 in Table IV.

TABLE IV Algorithm 4 Predict t_(λ)(λ*, λ*_(d)). 1: Input λ*_(d), λ* andthe uncertainty intervals [k, k], [τ, τ] and [T, T]; 2: Input ϵ; 3:Input N_(TV) and N_(T) _(s) ; 4: Run the MIMO simulator 3 times toobtain the computation time t_(β,1), t_(β,2) and t_(β,3); 5: t_(β) =(t_(β,1) + t_(β,2) + t_(β,3)) / 3; 6:${x = \frac{\log( {\epsilon/{\max( {{\overset{\_}{\lambda}}^{*} - 0.5_{1 \times m}} )}} )}{\log(0.5)}};$7: I_(λ) = ceil(x); 8:${y = \frac{\log( {\epsilon/( {{\overset{\_}{\lambda}}_{d}^{*} - 0.5_{1 \times m}} )} )}{\log(0.618)}};$9: I_(λ) _(d) = ceil(y); 10: t_(λ) = ((I_(λ) × N_(TV) + N_(T) _(s) ) ×2 + I_(λ) _(d) × (I_(λ) × N_(TV) + N_(T) _(s) )) × t _(β); 11: end

In Algorithm 4, N_(TV) and N_(T) _(s) (line 3) are the numbers ofextreme-case systems considered to calculate the total variation and thesettling time. According to Algorithm 1, both values can be eight bydefault, but the number of extreme-case systems considered in Algorithm1 can be optimized to further save computation time. Also, t_(β) (line5) is the time cost for a single run of the simulation (based on whichthe running time of Algorithm 1 can be obtained). As a computer may needmore time to run a program for the first time, the average of multipleruns (such as three) can be used as the simulation time. In addition,I_(λ) (line 7) and I_(λ) _(d) (line 9) are the iteration numbers forAlgorithm 2 and 3, respectively. The formula in line 10 is constructedbased on the structure of the tuning algorithm.

Industrial Example

In this section, the proposed tuning algorithm is illustrated withreference to an example extracted from a real application involving MDpaper machine control. The example illustrates the efficiency of theproposed algorithms. In particular, this example involves a 2-by-3process of a paper machine, where a controller uses a model to controlthe Conditioned Weight (CW) and Moisture (M) of paper products. Inputsto the process are Base Stock Flow (SF) to the headbox, Main Steam (MS)in the drying cylinders, and Machine Speed (S). The nominal model isshown in Table V below.

TABLE V O\I SF MS S CW $\frac{0.35}{{80s} + 1}e^{{- 80}s}$ —$\frac{- 0.025}{{30s} + 1}e^{{- 51}s}$ M$\frac{0.4}{{90s} + 1}e^{{- 80}s}$$\frac{- 0.08}{{150s} + 1}e^{{- 80}s}$ —This model was identified using an advanced industrial control softwarepackage. The MPC weighting matrices are set to Q₁=I, Q₂=0.01×I, andQ₃=0. The prediction horizon H_(p) is 68, and the control horizon H_(u)is 27. For the nominal model in Table V, the proposed tuning algorithmis applied at an uncertainty level [−50%, 30%] with OS*=20%. The tuningprocedure took 9.56 seconds on a desktop computer with an i5 processingcore and 6 GB of memory, and the predicted tuning time from Algorithm 4was 10.00 seconds. The obtained closed-loop step response envelopes aswell as the tuning results are shown in FIGS. 4A and 4B for all outputs.

To test the design results in a real-time MPC plus simulatorenvironment, to account for model mismatch, the actual process is takenas shown in Table VI, which lies within the uncertainty level [−50%,30%] of the nominal system in Table V.

TABLE VI O\I SF MS S CW $\frac{0.455}{{104s} + 1}e^{{- 104}s}$ —$\frac{- 0.0325}{{30s} + 1}e^{{- 66.3}s}$ M$\frac{0.52}{{117s} + 1}e^{{- 104}s}$$\frac{- 0.1040}{{195s} + 1}e^{{- 104}s}$ —The initial operating points are y(0)=[432; 8] and u(0)=[3790; 79; 447].The MPC optimization parameters H_(p), H_(u), Q₁, Q₂, and Q₃ are chosento be the same as those used for the above tuning results, and thefollowing constraints on control signals are incorporated in the MPCimplementation:0.9u(0)≤U≤1.1u(0),−0.1u(0)≤ΔU≤0.1u(0)  (16)

To take into account possible changes of operating conditions, asetpoint change of 1 lb/1000 ft² is made at t=100 s for all outputs, anoutput disturbance of 2 lbs/1000 ft² is introduced for output 1 at t=320s, an output disturbance of −2 lbs/1000 ft² is in effect at t=150 s foroutput 2, an input disturbance of 10 gpm is made at t=220 s for input 1,and measurement noise is taken to be zero-mean Gaussian with a standarddeviation 0.1 lbs/1000 ft². From FIGS. 5A and 5B, it is shown thatdespite the large model mismatch and measurement noise, all systemoutputs robustly track the setpoint for changes in working conditions,which further indicates the efficiency of the proposed tuning algorithm.

CONCLUSION

In this patent document, systematic procedures have been described forautomatic parameter tuning of model-based controllers used in MIMOprocesses under user-specified parametric uncertainties. An envelopealgorithm for time-domain closed-loop performance visualizationconsidering the parametric uncertainties for a MIMO process wasprovided. Also, a fast MIMO tuning algorithm that automaticallydetermines controller tuning parameters λ and λ_(d) that fulfill thespecifications on the worst-case time-domain performance indices underthe parametric uncertainty specifications for the MIMO process wasprovided. In addition, a technique to predict the tuning time wasprovided to further improve the user-friendliness of the overall tuningalgorithm.

Note that while the techniques and algorithms were often described abovewith reference to tuning an MD-MPC controller associated with a webmanufacturing or processing system, the same or similar techniques couldbe used to tune any other suitable model-based controller. Themodel-based controller may or may not be used in a web manufacturing orprocessing system since MPC and other model-based controllers can beused in a wide variety of industries. The same or similar techniquescould be used to tune any suitable model-based controller in anysuitable industry.

In some embodiments, various functions described above are implementedor supported by a computer program that is formed from computer readableprogram code and that is embodied in a computer readable medium. Thephrase “computer readable program code” includes any type of computercode, including source code, object code, and executable code. Thephrase “computer readable medium” includes any type of medium capable ofbeing accessed by a computer, such as read only memory (ROM), randomaccess memory (RAM), a hard disk drive, a compact disc (CD), a digitalvideo disc (DVD), or any other type of memory. A “non-transitory”computer readable medium excludes wired, wireless, optical, or othercommunication links that transport transitory electrical or othersignals. A non-transitory computer readable medium includes media wheredata can be permanently stored and media where data can be stored andlater overwritten, such as a rewritable optical disc or an erasablememory device.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The terms “application”and “program” refer to one or more computer programs, softwarecomponents, sets of instructions, procedures, functions, objects,classes, instances, related data, or a portion thereof adapted forimplementation in a suitable computer code (including source code,object code, or executable code). The terms “transmit” and “receive,” aswell as derivatives thereof, encompass both direct and indirectcommunication. The terms “include” and “comprise,” as well asderivatives thereof, mean inclusion without limitation. The term “or” isinclusive, meaning and/or. The phrase “associated with,” as well asderivatives thereof, may mean to include, be included within,interconnect with, contain, be contained within, connect to or with,couple to or with, be communicable with, cooperate with, interleave,juxtapose, be proximate to, be bound to or with, have, have a propertyof, have a relationship to or with, or the like. The phrase “at leastone of,” when used with a list of items, means that differentcombinations of one or more of the listed items may be used, and onlyone item in the list may be needed. For example, “at least one of: A, B,and C” includes any of the following combinations: A, B, C, A and B, Aand C, B and C, and A and B and C.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

What is claimed is:
 1. A method for controlling a web processing systemby a model-based industrial process controller that controls associatedprocesses of a web process facility in the web processing systemcomprising: obtaining information based on at least one sensor locatedin the web process facility communicating to the model-based industrialprocess controller through a network; identifying (i) uncertaintiesassociated with multiple time-domain parameters of a model and (ii)time-domain performance specifications for the model-based industrialprocess controller, the model mathematically representing amultiple-input, multiple-output (MIMO) industrial process; andgenerating multiple tuning parameters for the controller based on theuncertainties and the time-domain performance specifications, the tuningparameters including vectors of tuning parameters associated with thecontroller, each vector including values associated with differentoutputs of the industrial process, wherein generating the multipletuning parameters comprises: constructing multiple multiple-input,single-output (MISO) tuning problems based on a MIMO tuning problem,each MISO tuning problem associated with a different output of the MIMOindustrial process, and solving each MISO tuning problem to identify oneof the values in each vector, wherein solving each MISO tuning problemcomprises: identifying the values in a first of the vectors assumingfixed values in a second of the vectors; and identifying, the values inthe second vector that minimize settling times for the outputs of theMIMO industrial process; controlling, by the controller, the associatedprocesses of the web process facility in the web processing systemnetwork utilizing the multiple tuning parameters.
 2. The method of claim1, further comprising: generating a graphical display identifying (i)multiple expected step responses of the MIMO industrial process based onthe tuning parameters of the controller and (ii) an envelope around theone or more expected step responses based on the uncertaintiesassociated with the time-domain parameters of the model; wherein theenvelope is generated using different combinations of extreme values ofthe time-domain parameters, the extreme values based on theuncertainties of the time-domain parameters.
 3. The method of claim 2,wherein: the time-domain parameters comprise a process gain, a timeconstant, and a time delay for each input-output pair of the model; thetime-domain performance specifications comprise requirements related toworst-case overshoots, settling times, and total variations; and theuncertainties are specified as intervals in which the time-domainparameters lie.
 4. The method of claim 1, wherein: the first vectorcomprises a vector of reference tracking performance ratios, eachreference tracking performance ratio defining a ratio between a desiredclosed-loop reference tracking response time of the controller and anopen-loop response time for one output of the MIMO industrial process;and the second vector comprises a vector of disturbance rejectionperformance ratios, each disturbance rejection performance ratiodefining a ratio between a desired closed-loop disturbance rejectingresponse time of the controller and the open-loop response time of theone output of the MIMO industrial process.
 5. The method of claim 1,further comprising: predicting a tuning time of the controller prior togenerating the tuning parameters for the controller.
 6. The method ofclaim 1, wherein obtaining the information identifying the uncertaintiesand the performance specifications comprises receiving the informationfrom a user.
 7. An apparatus for controlling a web processing system bya model-based industrial process controller that controls associatedprocesses of a web process facility in the web processing systemcomprising: at least one memory configured to store information,obtained based on at least one sensor located in the web processfacility communicating to the model-based industrial process controllerthrough a network, identifying (i) uncertainties associated withmultiple time-domain parameters of a model and (ii) time-domainperformance specifications for the model-based industrial processcontroller, the model mathematically representing a multiple-input,multiple-output (MIMO) industrial process; and at least one processingdevice configured to generate multiple tuning parameters for thecontroller based on the uncertainties and the time-domain performancespecifications, the tuning parameters including vectors of tuningparameters associated with the controller, each vector including valuesassociated with different outputs of the industrial process, wherein themultiple tuning parameters are utilized by the controller to control theassociated processes of the web process facility in the web processingsystem, wherein the at least one processing device is configured to:construct multiple multiple-input, single-output IMISO) tuning problemsbased on a MIMO tuning problem, each MISO tuning problem associated witha different output of the MIMO industrial process; and solve each MISOtuning problem to identify one of the values in each vector, wherein theat least one processing device is configured to: identify the values ina first of the vectors assuming fixed values in a second of the vectors;and identify the values in the second vector that minimize settlingtimes for the outputs of the MIMO industrial process.
 8. The apparatusof claim 7, wherein the at least one processing device is furtherconfigured to: generate a graphical display identifying (i) multipleexpected step responses of the MIMO industrial process based on thetuning parameters of the controller and (ii) an envelope around the oneor more expected step responses based on the uncertainties associatedwith the time-domain parameters of the model; generate the envelopeusing different combinations of extreme values of the time-domainparameters, the extreme values based on the uncertainties of thetime-domain parameters.
 9. The apparatus of claim 8, wherein: thetime-domain parameters comprise a process gain, a time constant, and atime delay for each input-output pair of the model; the time-domainperformance specifications comprise requirements related to worst-caseovershoots, settling times, and total variations; and the uncertaintiesare specified as intervals in which the time-domain parameters lie. 10.The apparatus of claim 7, wherein: the first vector comprises a vectorof reference tracking performance ratios, each reference trackingperformance ratio defining a ratio between a desired closed-loopreference tracking response time of the controller and an open-loopresponse time for one output of the MIMO industrial process; and thesecond vector comprises a vector of disturbance rejection performanceratios, each disturbance rejection performance ratio defining a ratiobetween a desired closed-loop disturbance rejecting response time of thecontroller and the open-loop response time of the one output of the MIMOindustrial process.
 11. The apparatus of claim 7, wherein the at leastone processing device is further configured to: predict a tuning time ofthe controller prior to generation of the tuning parameters for thecontroller.
 12. A non-transitory computer readable medium containinginstructions that when executed cause at least one processing device tocontrol a web processing system by a model-based industrial processcontroller that controls associated processes of a web process facilityin the web processing system: obtain information based on at least onesensor located in the web process facility communicating to themodel-based industrial process controller through a network; identify(i) uncertainties associated with multiple time-domain parameters of amodel and (ii) time-domain performance specifications for a model-basedindustrial process controller, the model mathematically representing amultiple-input, multiple-output (MIMO) industrial process; and generatemultiple tuning parameters for the controller based on the uncertaintiesand the time-domain performance specifications, the tuning parametersincluding vectors of tuning parameters associated with the controller,each vector including values associated with different outputs of theindustrial process, wherein the multiple tuning parameters are utilizedby the controller to control the associated processes of the web processfacility in the web processing system, wherein the instructions thatwhen executed cause the at least one processing device to generate themultiple tuning parameters comprise instructions that when executedcause the at least one processing device to: construct multiplemultiple-input, single-output (MISO) tuning problems based on a MIMOtuning problem, each MISO tuning problem associated with a differentoutput of the MIMO industrial process; and solve each MISO tuningproblem to identify one of the values in each vector, wherein theinstructions that when executed cause the at least one processing deviceto solve each MISO tuning problem comprise instructions that whenexecuted cause the at least one processing device to: identify thevalues in a first of the vectors assuming fixed values in a second ofthe vectors, the first vector comprising a vector of reference trackingperformance ratios, each reference tracking performance ratio defining aratio between a desired closed-loop reference tracking response time ofthe controller and an open-loop response time for one output of the MIMOindustrial process; and identify the values in the second vector thatminimize settling times for the outputs of the MIMO industrial process,the second vector comprising a vector of disturbance rejectionperformance ratios, each disturbance rejection performance ratiodefining a ratio between a desired closed-loop disturbance rejectingresponse time of the controller and the open-loop response time of theone output of the MIMO industrial process.
 13. The non-transitorycomputer readable medium of claim 12, wherein the instructions whenexecuted further cause the at least one processing device to: generate agraphical display identifying (i) multiple expected step responses ofthe MIMO industrial process based on the tuning parameters of thecontroller and (ii) an envelope around the one or more expected stepresponses based on the uncertainties associated with the time-domainparameters of the model; generate the envelope using differentcombinations of extreme values of the time-domain parameters, theextreme values based on the uncertainties of the time-domain parameters.14. The non-transitory computer readable medium of claim 13, wherein:the time-domain parameters comprise a process gain, a time constant, anda time delay for each input-output pair of the model; the time-domainperformance specifications comprise requirements related to worst-caseovershoots, settling times, and total variations; and the uncertaintiesare specified as intervals in which the time-domain parameters lie. 15.The non-transitory computer readable medium of claim 12, wherein theinstructions when executed further cause the at least one processingdevice to: predict a tuning time of the controller prior to generationof the tuning parameters for the controller.